Greenhouses are notorious wasters of heat in cold climates, which is expensive to the point where most winter produce is trucked in from tropical regions. Most greenhouses suffer the fundamental problem that if light can come into a window in the daytime, infrared heat can go out the same window at night and during cloudy weeks in winter.
Standard glass greenhouses often have a single layer of glass over the north and south roof of the greenhouse. This design often leaks heat through the glass, both convective heat and radiated heat. Single pane glass has an insulating value of perhaps R-1, where insulated walls might have an insulating value of R-20 or R-30. Also, many glass windows leak air around their edges.
A number of methods have been used to reduce heat losses in greenhouses. Low-emissivity glass blocks the outward and inward passage of infrared rays through the glass. Double or triple glazing, or some combination of glass and plastic windows, reduces convection losses. A double glazed window can have an insulating value of R-2.5.
Hoop houses have two layers of impermeable vinyl chloride kept apart by inflation with an air pump. While having two layers of plastic reduces heat losses and eliminates air leaks, heat losses at night and on cloudy days are still substantial. Also, the vinyl sheets decay after about four years and become toxic waste. Neither hoop houses nor classic greenhouses are particularly immune from the ravages of heavy snow, large hail and hurricanes. Kalwall, a fiberglass-plastic composite, lasts about 10 years before it becomes waste.
One window maker has constructed double pane window walls where the space between the panes can be filled with Styrofoam pellets at night. This device does reduce heat losses at night, but requires equipment to blow the Styrofoam pellets in and out. The problem of gathering sparse sunlight on windy, mostly cloudy days with in-and-out sun and clouds remains unsolved.
One promising solution is to focus and concentrate light through a small window and then spread the light out on the inside. As shown in Item A, U.S. Pat. No. 4,198,953 uses a Fresnel lens to get light through a small aperture into a greenhouse. However, this particular device requires that the Fresnel lens track the sun hourly. Furthermore, the Fresnel lens is in a vulnerable wind position, the windows are placed facing the sky so that the window might leak or be covered with dry leaves or other flammable materials, and the lens might become snow-covered.
At least two inventors have designed reflective linear troughs with end reflectors on the roofs of houses to concentrate sunlight into a linear target window. As shown in Item B, in U.S. Pat. No. 4,257,400 a section of a parabolic trough has been built on a house roof. This design keeps its target window nearly vertical, somewhat preventing rain from seeping in. On tree-shaded lots, the design's basic idea of gathering light from the roof is an improvement from gathering light from the building's south side. However, this particular design has snow shoveling and ice collection problems or can be covered with leaves, and the reflector could still be somewhat shaded by taller trees and buildings to the south. Finally, the upward-facing mirrored surface is angled to be less efficient per square foot of reflector in winter when heat and light is most needed, and more efficient in summer.
Another parabolic trough designed into a house roof, as shown in Item C, is demonstrated in U.S. Pat. No. 4,111,360, which generates plenty of heat but has several possible problems. Snow and leaves can accumulate on the flat part of the mirrored roof. Tall trees to the south of the house can block the sun from reaching the lower part of the roof. The collector pipe is up in the air, creating insulation costs for the collector pipe. Any heat leaking through the collector pipe's insulation is lost. Direct sunlight can't be gotten into the house for lighting—only heat from the collector pipe's fluid can be actively pumped into the house. If the collector's pump should ever fail then the collector might overheat and be damaged. The collector shown requires the construction of an overbuilt house to avoid tearing apart the house's steep peak in sustained hurricane force winds. Near sunrise and sunset, the trough's focal point is far to the east or west of the collector pipe, possibly focusing on a neighbor's roof or on a nearby tree.
If a greenhouse can be built which protects warm weather crops through the winter, a house or a commercial building could equally be built on the same principles. Currently 10% of the world's fossil fuel annual supply is used to provide heat for buildings.